1. Field of the Invention
The present invention relates to a method and a device for driving an organic EL display device employing an organic electroluminescence light emitting element (hereinbelow, referred to as organic EL element).
2. Description of the Related Art
Organic EL display devices, which employ an organic EL panel having a structure that respective organic EL elements are disposed at respective pixels of matrix electrodes, have been realized. Such an organic EL panel comprises a substrate, such as a glass substrate, a plurality of anode strips (hereinbelow, referred to as the anode electrodes) disposed thereon and a plurality of cathode strips (hereinbelow, referred to as the cathode electrodes) disposed thereon so as to extend in a direction perpendicular to the anode electrodes, the anode electrodes comprising a transparent conductive layer, such as an ITO film, and being connected to an anode or forming an anode per se, the cathode electrodes comprising a metal film connected to a cathode or forming a cathode per se. The intersection between an anode electrode and a cathode electrode forms a pixel, and an organic thin film (organic EL element) is sandwiched between both electrode. Thus, pixels, each of which comprises an organic EL element, are disposed so as to have a matrix pattern in a planar fashion on the substrate.
An organic EL element has similar characteristics to a semiconductor light emitting diode. In other words, an organic EL element emits light when a certain voltage is applied across both electrodes to supply a current to the organic EL element in such a state that an anode side serves as a high voltage side. Specifically, when the difference between the potential on the anode side and the potential on the cathode side is beyond a turn-on-voltage, a current starts flowing through the organic EL element. Conversely, when the cathode side is placed at a high potential, the organic EL element emits no light since no almost current flows. For this reason, an organic EL element is called an organic LED in some cases.
An organic EL panel may be driven by passive matrix addressing. When an organic EL panel is driven, the anode electrodes and the cathode electrodes of the organic EL panel may be set as scanning electrodes or data electrodes. In other words, the anode electrodes and the cathode electrodes may serve as scanning electrodes and data electrodes, respectively, or the anode electrodes and the cathode electrodes may serve as data electrodes and scanning electrodes, respectively. Explanation will be made with respect to a case wherein the cathode electrodes and the anode electrodes serve as scanning electrodes and data electrodes, respectively. For this reason, the cathode electrodes will be called scanning electrodes, and the anode electrodes will be called data electrodes.
When an organic EL panel may be driven by passive matrix addressing, the scanning electrodes are connected to a scanning electrode driver with a constant-voltage circuit, providing the scanning electrodes with constant-voltage drive. The scanning electrodes are sequentially scanned so that one of the scanning electrodes is put in a selected state with a selection voltage applied, and the remaining scanning electrodes are put in a non-selected state without the selection voltage applied. In general, scanning is sequentially performed so that a selection voltage is applied to a scanning electrode in each selection period, starting from an endmost one of the scanning electrode and ending at the other endmost one of the scanning electrodes. All scanning electrodes are scanned in a certain period of time to apply a certain driving voltage to a selected pixel.
On the other hand, the data electrodes are connected to a data electrode driver with a constant-current circuit (constant-current source). A display data that corresponds to a display pattern of selected scanning electrodes is supplied to all data electrodes in synchronization with scanning. A current pulse that has been supplied to the data electrodes from the constant-current circuit flows into a selected scanning electrode through the organic EL element disposed at the intersection between the selected scanning electrode and the opposing data electrode.
A pixel comprising an organic EL element emits light only during a period of time wherein the scanning electrode connected to the pixel is selected while a current is supplied to the pixel from the opposed data electrode. When supply of the current from the data electrode is stopped, light emission is also stopped. All scanning electrodes are sequentially and repeatedly scanned by supplying a current to organic EL elements sandwiched between the data electrodes and the scanning electrodes in this way. In accordance with a desired display pattern, the emission and the non-emission of light is controlled with respect to the pixels in the entire display screen.
The scanning electrode driver sets the potential of a selected scanning electrode at a lower level than that of a non-selected scanning electrode. It is assumed that the potential of a selected scanning electrode is a selection voltage VCOML and that the potential of a non-selected scanning electrode is a non-selection voltage VCOMH. In most of cases, ground potential is utilized as the selection voltage VCOML. Data electrodes that contain no pixels to emit light in a selected row are set at a certain potential (hereinbelow, referred to as VCL). The potential VCL is set so that the difference (VCL−VCOML) between the potential VCL and the selection voltage VCOML is lower than the turn-on-voltage. In most of cases, the potential VCL is set at ground potential. The data driver also sets the potential of data electrodes that contain pixels to emit light in a selected row, and a current flows from such data electrodes into a selected scanning electrode. The potential of such data electrodes is set so as to flow a constant current. However, it is not allowable to set the potential of the data electrodes at a higher level than the supply voltage VSEG of the constant-current circuit. An array of pixels, which extends in parallel with the scanning electrodes is called a “row” while an array of pixels, which extends in parallel with the data electrodes, is called a “column”.
An organic EL element has temperature characteristics wherein the turn-on-voltage lowers as the temperature increases. In some cases, temperature compensation is made so as to reduce power consumption in the data electrode driver by lowering the supply voltage VSEG at a high temperature (see, e.g., JP-A-2003-150113, paragraphs 0023 to 0026, and FIGS. 1 and 3).
FIG. 11 is a block diagram showing the drive circuit of a conventional organic EL display device described in the reference stated earlier. In the structure shown in FIG. 11, a plurality of data electrodes 110 and a plurality of scanning electrodes 111 are disposed so as to be perpendicular to each other in an organic EL panel 101. Each organic EL element is shown as a diode. A scanning electrode driver 102 includes a scanning switch with respect to each of the scanning electrodes 111, the scanning switches providing scanning electrodes with either one of ground potential as the selection voltage VCOML and a reverse-bias voltage (non-selection voltage) generated by a second temperature compensation circuit 106.
A data driver 103 includes a constant-current circuit and a driving switch with respect to each of the data electrodes 110, the constant-current circuit introducing a supply voltage VSEG from a supply circuit 105b and supplying a constant current to the relevant data electrode, and the driving switch putting the relevant data electrode 110 in either one of a supply state to supply a current to the relevant data electrode 110 from the relevant constant-current circuit and a non-supply state to supply no current to the relevant data electrode from the relevant constant-current circuit. A controller 104 not only controls the scanning electrode driver 102 so as to sequentially apply the selection voltage VCOML to the respective scanning electrodes 111 but also outputs a data to the data electrode driver 103, the data corresponding to pixels in a row relevant to a scanning electrode 111 with the selection voltage VCOML applied thereto. The data electrode driver 103 determines the respective states of the drive switches according to an input data.
The supply circuit 105b receives, from a temperature detecting means 105a comprising a thermistor, a signal in response to the ambient temperature of the organic EL elements. The supply circuit 105b generates the supply voltage VSEG at a level in response to the ambient temperature of the organic EL elements and applies the supply voltage as the driving voltage to organic EL elements through the data electrode driver 103. The temperature detecting means 105a and the supply circuit 105b form a first temperature compensation circuit 105. The second temperature compensation circuit 106 introduces the supply voltage VSEG from the supply circuit 105b, generates the non-selection voltage VCOMH at a lower level than the value of the supply voltage VSEG by a certain amount, and supplies the VCOMH to the scanning electrode driver 102.
FIG. 12 is a schematic view showing a relationship between an ambient temperature, a supply voltage VSEG (corresponding to T1 in this figure) and a non-selection voltage VCOMH (corresponding to T2 in this figure) described in the reference stated earlier. In FIG. 12, the horizontal axis represents a temperature (° C.), and the vertical axis represents a voltage (V) Based on an ambient temperature of the organic EL elements detected by the temperature detecting means 105a, the supply circuit 105b lowers the supply voltage VSEG as the ambient temperature increases, which is shown in FIG. 12. The second temperature compensation circuit 106 sets the non-selection voltage VCOMH at a voltage that is lower than the supply voltage VSEG by a certain offset amount×(3V in the example shown in FIG. 12).
In the reference stated earlier, it is described that by lowering the supply voltage VSEG as the ambient temperature increases, the supply voltage VSEG is prevented from being supplied to the data electrode driver 103 at an unnecessarily high level at a high ambient temperature, avoiding an increase in the consumption power of the data electrode driver 103. It is also described that by lowering the non-selection voltage VCOMH as the ambient temperature increases, an organic EL element is prevented from emitting light in a non-selected state (when the non-selection voltage VCOMH is applied to the scanning electrode 111 of the organic EL element) because of a decrease in the turn-on-voltage of the organic EL element caused by an increase in the ambient temperature.